1. Field of the Invention
This invention is directed to a substrate carrying thereon a coating comprising light metal fluorides and a preferred process for forming same. The preferred process comprising using sol-gel techniques to form the coating which may comprise a concentration gradient of the light metal fluorides across the thickness of the coating to provide it with unique anti-reflective properties over wide viewing angles.
2. Discussion of the Related Art
Various attempts have been made in the art to modify the amount of light reflected by substrates. Uncoated window or windshield glass with an index of refraction of about 1.5 reflects a portion of the light incident upon it. At normal (perpendicular) incidence, the reflection is 4 percent per surface, yielding an 8 percent reflection by the glass (considering the two surfaces of the glass). For this uncoated glass, the reflection coefficient increases to about 15 percent at a 60.degree. angle tilt of the glass with respect to the vertical and to 26 percent at a 70.degree. tilt with respect to the vertical. Since in automotive glass applications (e.g., windshields), the glass often displays tilt angles as high as 70.degree. from the vertical, the reflection coefficient of the glass is undesirably high. Current practice is to modify the reflectivity of the glass by providing the glass with anti-reflection coatings made of single or multiple films. Prior art coatings may provide the glass with reflection coefficients as low as 1/2 percent when the glass is positioned vertically and the incident light is of a particular wave length. But, as one tilts the angle of the glass, that is, angles it with respect to the vertical, the reflection coefficient of the coated glass increases even when such anti-reflective coatings are used. For example, when using a conventional triple layer coating comprising a first layer of a mixture of titanium oxide and silicon oxide, a second layer of titanium oxide and a third layer of silicon oxide, the reflection coefficient of the coated glass increases to about 8.4 percent as the glass is positioned at 6.degree. with respect to the vertical and to 21 percent as the glass is Positioned at a 70.degree. angle with respect to the vertical. In using a conventional single layer coating comprising, for example, fused silica, the reflection coefficient of the coated glass increases to about 11 percent for a glass angled at 60.degree. with respect to the vertical and to 23 percent for a glass positioned at 70.degree. with respect to the vertical. In all of these cases, the reflectivity coefficient increases substantially if one changes the wave length (color) of the incident light. It is highly desirable in automotive applications to utilize coatings for glass which lower the reflection coefficient even of highly tilted glass such as a windshield at substantially all colors of the visible spectrum of incident light.
U.S. Pat. No. 2,337,460 discloses reducing the reflection from glass surfaces by forming upon the surface of the glass films rich in calcium fluoride and of such thickness as to inhibit or reduce the reflection from the surface. According to the reference, the films are formed by first treating the glass with a strong mineral acid to leach out soluble alkali and alkali earth compounds and leave a surface film rich in calcium and silica compounds. It is further taught therein that the surface is then treated with hydrogen fluoride in vapor phase to remove silica and to convert the calcium compounds into a film of calcium fluoride. One disadvantage with such a process is that by treating the glass surface with the acid, a haze is formed on the glass surface which reduces the optical quality of the glass. Further, the thin calcium fluoride layer formed on the glass according to that technique will have less than desirable durability. U.S. Pat. No. 3,176,574 discloses a two layer coating taught useful to provide low reflectivity with respect to all colors of the visible spectrum and a method of making the coating. It includes an outer layer of a low index low reflection material such as magnesium fluoride and an inner layer comprising a high index material such as lanthanum oxide. The index of refraction of the inner layer on the substrate is graded from low to high index of refraction proceeding outwardly from the glass. This reference teaches that the index of refraction of the inner layer may be graded by varying the oxygen content of this layer when it is applied by high temperature, vacuum evaporation techniques. One of the difficulties with making a graded index film as disclosed in the patent is that critical conditions are required to form a graded index by the vacuum evaporation techniques disclosed therein. In U.S. Pat. No. 3,738,732 anti-reflective coatings are disclosed which comprise at least three layers of materials deposited in vacuum, each layer having a defined optical thickness and refractive index. U.S. Pat. No. 4,535,026 discloses silica coatings which are graded in porosity in order to be substantially non-reflecting over a wide band of radiation. In order to form such a coating, a porous silica layer is first formed on the substrate and then the silica layer is etched in order to enlarge the pores in a graded fashion, being more porous near an exposed surface. The index of refraction of the layer is taught to vary so that the index of refraction of the exposed surface of the layer approaches that of air and so that the index of refraction of the layer surface nearest the substrate approaches the index of refraction of the substrate. Such techniques, however, result in coated substrates which are of less than desirable optical quality and durability, since impurities can be trapped in the pores. U.S. reissued patent 22,076 discloses a method to deposit a coating of magnesium fluoride onto a substrate by evaporation. As with all the references disclosed above, the thickness of the film is related to a wave length of the light to be reflected and these coatings will generally only reduce reflection of incident light near that particular wave length.
Additionally, while prior art coatings may provide anti-reflective properties for a substrate such as glass which is positioned vertically, the anti-reflective properties of the glass are substantially diminished when the glass is positioned at a large angle with respect to the vertical.